1. Field of the Invention
The present invention relates to a method and apparatus for automatically adjusting white balance or, in particular, to a method and apparatus for automatically adjusting white balance using and internal light measurement system.
2. Description of the Related Art
Conventionally, an automatic white balance adjusting method includes an internal light measurement system in which the white balance can be adjusted by means of the light that is striking an object. In this internal light measurement system, when the white balance is correct, it is assumed that the average of the color difference signals on the entire screen frequently provides a given reference level showing white or grey, and therefore, the gains of the signals of red/blue are feedback controlled so that the average of the color difference signals (E.sub.R -E.sub.Y) and (E.sub.B -E.sub.Y) becomes the above-mentioned reference level.
However, in the above system, when the above-mentioned assumption does not hold, for example, in the case of an object with a blue sky and a blue sea for a background, or in the case of an object with a red wall for a background, color difference signals over the entire screen if they are integrated (or averaged), provide blue or red, not grey. If the above-mentioned white balance adjustment is made on such objects, then the backgrounds are caused to fade and the colors of a figure and the like as main objects are controlled in the direction of a complementary color (strong color), resulting in a so called color failure.
In view of this, in order to solve the above-mentioned problem, the present applicant filed an application in Japan for a patent (Japanese Patent Application No. 62-202509 and the like) relating to an automatic white balance adjusting method in which strong colors (chromatic colors) are removed from an original picture and only the color information on the portion of achromatic colors such as white, grey and the like within the screen is integrated, thereby eliminating the fading of the background and the miscontrol of main object or the above-mentioned color failure.
A description will be given below of the above-mentioned automatic white balance adjusting method.
First of all it is assumed that, from a picture containing a strong red color as shown in FIG. 5(A), a color difference signal (E.sub.R -E.sub.Y) as shown in FIG. 5(B) is obtained. When this color difference signal is integrated over the entire screen, the resultant average value is moved in the red direction from a black level. For this reason, if the above-mentioned automatic white balance adjustment is made, then the color of the face of a man is miscontrolled in the direction of the complementary color (cyan) of red.
In view of this, in order to blank the color difference signal, showing the strong red, out of the picture in FIG. 5(A), there is established a certain level (a threshold level) and the strong color difference signals exceeding the threshold level are blanked to a black level (black level of a reference color temperature ) as shown in FIG. 5 (C). As a result of this, the strong color difference signal is removed from the color difference signal of the original picture so that the average value of the color difference signal can be made to approach a grey color.
Referring now to FIG. 4, there is shown a block diagram of an example of an automatic white balance adjusting apparatus employing the above mentioned principle. As shown in FIG. 4, the light that enters through a lens 10 is photoelectrically converted to signals of red, green, and blue (E.sub.R, E.sub.G, E.sub.B) by means of an image pickup element 12. The signals E.sub.R, E.sub.G, E.sub.B are respectively passed through amplifiers 14, 16, 18 and are then fed to a matrix circuit 20, where the signals are converted to a brightness signal E.sub.Y and color difference signals (E.sub.R -E.sub.Y), (E.sub.B -E.sub.Y) and then are input to an encoder 22. In the encoder 22, the color difference signals are rectangular two-phase modulated and are then added to the brightness signal E.sub.Y. The thus modulated and added signals are fed out as video signals in the NTSC system from the encoder 22.
On the other hand, the color difference signal (E.sub.R -E.sub.Y) and (E.sub.B -E.sub.Y) are fed to a blanking control part 40.
The blanking control part 40 composed of an upper threshold level setting means 41A for the color difference signal (E.sub.R -E.sub.Y), a lower threshold level setting means 41B, comparators 42A, 42B, an AND circuit 43, a blanking switch 44, an upper threshold level setting means 45A for the color difference signal (E.sub.B -E.sub.Y), a lower threshold level setting means 45B, comparators 46A, 46B, an AND circuit 47, and a blanking switch 48.
The upper and lower threshold level setting means 41A and 41B are respectively used to generate upper and lower threshold levels for blanking strong color difference signals on the red and cyan sides of the color difference signal (E.sub.R -E.sub.Y), and then also output the upper and lower threshold levels, respectively.
The upper threshold level and the color difference signal (E.sub.R -E.sub.Y) are respectively applied to the positive and negative inputs of the comparator 42A. The comparator 42A compares these two input signals to output an H level signal when the color difference signal (E.sub.R -E.sub.Y) is smaller than the upper threshold level and to output a strong color difference detection signal (an L level signal) when the color difference signal is greater than the upper threshold level. Also, to the positive and negative inputs of the comparator 42B are applied the color difference signal (E.sub.R -E.sub.Y) and the lower threshold level, respectively. The comparator 42B compares these two input signals and outputs the H level signal when the color difference signal (E.sub.R -E.sub.Y) is greater than the lower threshold level while it outputs the strong color difference detection signal (L level signal) when the color difference signal is smaller than the lower threshold level.
The AND circuit 43 outputs an H level signal when both of the two input signals from the comparators 42A, 42B are both H level signals, that is, when the color difference signal (E.sub.R -E.sub.Y) is present between the upper and lower threshold levels, while it outputs an L level signal when at least one of the two input signals is an L level signal, that is, when the color difference signal (E.sub.R -E.sub.Y) goes beyond the upper or lower threshold level.
The blanking switch 44 comprises an input contact 44A to which the color difference signal (E.sub.R -E.sub.Y) is input, and input contact 44B to which a reference level from a reference level setting means 32 is input, and a movable contact 44C which can be selectively connected to one of the contacts 44A, 44B. When the H level signal is input from the AND circuit 43, the blanking switch 44 connects the movable contact 44C to the contact 44A and outputs the color difference signal (E.sub.R -E.sub.Y) to an integrating circuit 24. When the level signal is input from the AND circuit 43, the blanking switch 44 connects the traveling contact 44C to the contact 44B and outputs the reference level to the integrating circuit 24.
On the other hand, the upper and lower threshold level setting means 45A and 45B are used to set up the upper and lower threshold levels which are respectively used to blank a strong color difference signal on the blue side of the color difference signal (E.sub.B -E.sub.Y) and a strong color difference signal on the yellow side thereof. Also, the comparators 46A, 46B, AND circuit 47 and blanking switch 48 can be operated similarly to the above-mentioned comparators 42A, 42B, AND circuit 43 and blanking switch 44 and, therefore, their detailed description is omitted here.
As can be understood from the foregoing description, the color difference signals (E.sub.R -E.sub.Y) and (E.sub.B -E.sub.Y) are integrated by the integrating circuits 24 and 26, respectively. In such integration, when the color difference signal provides a strong color difference signal going beyond the upper or lower threshold level, the strong color difference signal is blanked before the color difference signal is integrated, that is, the reference level is integrated in place of the strong color difference signal.
The integration average values integrated by the above-mentioned integrating circuits 24 and 26 are then input to the negative inputs of difference amplifiers 28 and 30, respectively. Also, to the positive inputs of the difference amplifiers 28 and 30 there are input from reference level setting means 32 and 34 reference levels which are, respectively, the average values of the color difference signals (E.sub.R -E.sub.Y) and (E.sub.B -E.sub.Y) when the average value of the colors over the entire screen shows a grey color. Thus, the difference amplifier 28 outputs to the amplifier 14 a gain control signal R .sub.CONT which can be obtained by amplifying a difference between the above-mentioned two input signals, thereby controlling the gain of a primary color signal E.sub.R which is amplified by the amplifier 14. Also, the difference amplifier 30 outputs to the amplifier 18 a gain control signal B .sub.CONT which can be obtained by amplifying the difference between the above-mentioned two input signals, thereby controlling the gain of a primary color signal E.sub.B to be amplified by the amplifier 18. By means of this, the primary color signals E.sub.R, E.sub.B are controlled in such a manner that the average values of the color difference signals (E.sub.R -E.sub.Y), (E.sub.B -E.sub.Y) over the entire screen can coincide with the reference levels, respectively.
However, since the color difference signals (E.sub.R -E.sub.Y) and (E.sub.B -E.sub.Y) vary greatly with respect to the variations of the color temperature, the width of the threshold levels in the above-mentioned threshold level setting means 41A, 41B, 45A, 45B must be comparatively greater and, as a result of this, the strong color cannot be removed to a sufficient degree. Also, if the width of the threshold level is made narrow, then even the portion of the signal that should not be blanked is blanked, which is a disadvantage.
On the other hand, it is possible to make the threshold level width comparatively smaller by finding the average value of the color difference signal over the entire screen and moving the threshold level parallel only by the variation of the above average value that varies according to the variation of the color temperature. In this case, however, other problems arise; for example, a circuit for setting the threshold level becomes complicated.